1. Field of the Invention
This invention relates to drive means for all-wheel-driven motor vehicles comprising an interaxle differential which is interconnected between a front axle and a rear axle and comprises three torque-transmitting members consisting of an input member and two output members, and a releasable, preferably non-posivitve coupling interconnected between two of said members, said differential and coupling being adapted to maintain a ratio of the torque applied to the front axle to the torque applied to the rear axle in excess of the ratio of the dynamic axle load on the front axle to the dynamic axle load on the rear axle under normal running conditions of the vehicle, i.e., when the vehicle is running substantially straight ahead without wheel spinning or wheel locking.
2. Description of the Prior Art
The running characteristics of an all-wheel-driven motor vehicle comprising an interaxle differential interconnected between the front axle and the rear axle are decisively influenced by the distribution of the torques for driving the two axles. The vehicle will be understeered when the torque for driving the front axle, related to the dynamic axle load on the front axle, exceeds the torque for driving the rear axle, related to the dynamic axle load on the rear axle. An oversteering can be achieved in that the torque for driving the rear axle is relatively increased. An understeered vehicle can be more easily driven by an ordinary driver. An oversteered vehicle is preferred for a sportive driving.
It is already known (see FIG. 18 of British Patent Specification 1,357,106) to provide a liquid friction coupling, which constitutes a non-positive coupling that is associated with an interaxle differential interconnected between a front axle and a rear axle. When there is no wheel spinning, the torque ratio lies between a fixed value, which is determined by the design of the interaxle differential, and a ratio which is obtained in dependence on the dynamic distribution of the weight to the two axles.
In another known arrangement (see FIG. 17 of British Patent Specification 1,357,106), one of the two axles is non-rotatably connected to the output member of the speed-changing transmission driven by the engine, and the second axle is driven by means of a liquid friction coupling. In an arrangement in which the latter axle is the front axle whereas the rear axle is rigidly connected to the drive, the ratio of the torques for driving the two axles will be in a range between a transmission of torque only to the rear axle and a ratio which depends on the rati of the dynamic axle loads, provided that there is no excessive wheel slip.
Whereas the two known drive means discussed hereinbefore operate to vary the distribution of the torques for driving the two axles, that variation is effected only in a rather restricted range having limits which are defined, on the one hand, by the condition in which only the rear axle is driven and, on the other hand, on the torque distribution effected by the interaxle differential, and are also determined by the requirement that the torque ratio must exceed the ratio of the dynamic axle loads. A reversal of that relationship is not possible although this would be desired for a sportive driving.